Evolution of the Discs large gene family provides new insights into the establishment of apical epithelial polarity and the etiology of mental retardation.

Cell polarity is essential to the function of many cell types, such as epithelial cells and neurons. The Discs large (Dlg) scaffolding protein was identified in Drosophila as a major regulator of basolateral epithelial identity. Four Dlg orthologs (Dlg1 through 4) are found in vertebrates, and mutations in the human Dlg3 gene are associated with X-linked mental retardation. We recently found that Dlg3 controls apical epithelial polarity and tight junction formation and contributes to neural induction in mouse development.(1) During evolution, Dlg3 acquired specific PPxY motifs, which bind to the WW domains of the E3 ubiquitin ligases, Nedd4 and Nedd4-2. This interaction results in monoubiquitination of Dlg3, leading to directed microtubule-dependent protein trafficking, via the exocyst complex, in different polarized cell types. Directed trafficking of Dlg3 plays an important role, during both mammalian development and in adulthood, in the establishment and maintenance of specialized apical cell junctions, such as tight junctions in epithelial cells and synapses in neurons.

Dlg3 trafficking and apical tight junction formation is regulated by nedd4 and nedd4-2 e3 ubiquitin ligases.

The Drosophila Discs large (Dlg) scaffolding protein acts as a tumor suppressor regulating basolateral epithelial polarity and proliferation. In mammals, four Dlg homologs have been identified; however, their functions in cell polarity remain poorly understood. Here, we demonstrate that the X-linked mental retardation gene product Dlg3 contributes to apical-basal polarity and epithelial junction formation in mouse organizer tissues, as well as to planar cell polarity in the inner ear. We purified complexes associated with Dlg3 in polarized epithelial cells, including proteins regulating directed trafficking and tight junction formation. Remarkably, of the four Dlg family members, Dlg3 exerts a distinct function by recruiting the ubiquitin ligases Nedd4 and Nedd4-2 through its PPxY motifs. We found that these interactions are required for Dlg3 monoubiquitination, apical membrane recruitment, and tight junction consolidation. Our findings reveal an unexpected evolutionary diversification of the vertebrate Dlg family in basolateral epithelium formation.